Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell has a membrane-electrode-assembly (MEA) that generally includes a thin, solid polymer membrane-electrolyte having an electrode having a catalyst on both faces of the membrane-electrolyte.
The MEA generally includes porous conductive materials, also known as gas diffusion media (GDM), which distribute reactant gases to the anode and cathode electrode layers including a finely divided catalyst supported on carbon particles and admixed with a proton conductive resin. The catalyst is typically a precious metal, for example, platinum. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Individual PEM fuel cells are typically connected in series, or stacked one on top of the other, to form what is referred to as a fuel cell stack. The quantity and type of fuel cells in a fuel cell stack may be selected to provide a fuel cell stack capable of providing a desired amount of electricity, for example, an amount of electricity sufficient to power an automotive vehicle.
At sub-freezing temperatures, e.g. temperatures below 0° C., starting the fuel cell stack is known to be more difficult than starting the fuel cell stack at higher temperatures, e.g. 25° C. Frozen water forms in the fuel cell stack at sub-freezing temperatures and may inhibit the flow of reactants through the fuel cell stack. Additionally, the ionic conductivity of the MEA is significantly reduced at sub-freezing temperatures.
To overcome the difficulties associated with starting a fuel cell stack in sub-freezing temperatures, it is known to provide supplemental heating. For example, electrical heaters have been employed to heat a coolant fluid, which is subsequently circulated through the fuel cell stack. However the addition of an effective electrical heater to the coolant system adds undesirable mass and volume to the fuel cell system.
Current practices also include adding heat to a fuel cell power system by exothermically reacting hydrogen with cathode air on the MEA cathodes of the fuel cell stack. However, cathode catalysts in PEM fuel cells are typically not optimized for hydrogen combustion at low temperatures, and the long term use of catalytic combustion of hydrogen and air on the cathode may affect the durability of the catalyst and catalyst support materials. As reported by Standke et al. in U.S. Pat. No. 7,135,245, a separate catalytic combustor is also known that lies adjacent to a fuel cell stack and includes a series of catalyst coated flow channels. The catalytic combustor may radiate heat to the fuel cell stack or circulate hot exhaust gas around the fuel cell stack.
Emissions from catalyst combustion can undesirably include uncombusted hydrogen. To minimize hydrogen emissions, catalytic combustion systems desirably have a rapid “light-off.” As used herein, the term “light-off” refers to a rate at which the catalyst reaches a temperature where the rate of reaction on the catalyst surface becomes diffusion-limited instead of kinetics-limited. At light-off, the catalyst combustion of hydrogen and oxygen occurs rapidly. With a rapid light-off, the catalyst rapidly heats to the light-off temperature. However, the presence of water on the catalyst of the catalytic combustor is known to be detrimental to light-off of combustion catalysts. Water acts to cool the catalyst surface and reduce a surface area of the catalyst available for reaction.
There is a continuing need for a fuel cell system that provides supplemental heating of a fuel cell stack that improves low temperature performance and start time of a vehicle employing the fuel cell stack without relying on power from the fuel cell stack. A system that improves a reliability of the fuel cell stack by providing supplemental heating, as well as removing exothermal hydrogen-air reactions from the cathode of the fuel cell stack and reduces hydrogen emissions, is also desired.